The present invention generally relates to modem units, and more particularly to a modem unit which enables a data exchange between personal computers and the like, via a telephone line.
The size and thickness of portable personal computers, portable word processors and the like have been reduced by recent technological development, and they continue to be further reduced. Additionally, data communications between computers via telephone lines have also become popular.
When making such data communications, a modem converts a digital signal into an analog signal and transmits the analog signal to the telephone line. The modem also receives the analog signal from the telephone line and converts the analog signal into a digital signal. In order to guarantee separation or isolation of the telephone line and a power source part of a data terminal equipment such as a computer, a line transformer is provided in the modem. For example, the DC line withstand voltage is set to 250 V and the isolation resistance is set to 1 MOhms or greater in Japan. The line withstand voltage may vary depending on the region or country, and other examples of the line withstand voltage are 1000 V.sub.nns (at AC 60 Hz) and 3.5 kV. The line transformer provides a coupling between the telephone line and the data terminal equipment with respect to the AC signal, and also provides DC isolation between the telephone line and the data terminal equipment.
The line transformer is bulky. For this reason, a conventional modem which uses the line transformer cannot be made thin and compact. Hence, it has been impossible to provide the conventional modem within a portable computer, an integrated circuit (IC) card and the like.
A description will now be given of the characteristics required of the modem, before explaining the construction of the conventional modem.
The following characteristics are required of a modem connected to a telephone line.
First, a circuit is required to form a DC current loop for capturing the telephone line. For example, such a circuit has a resistance of 50 to 300 Ohms at a current of 20 to 120 mA.
Second, it is necessary to isolate the telephone line and the modem unit with a high resistance with respect to the DC signal. For example, the high resistance must be 1 MOhms or greater at a DC voltage of 250 V.
Third, the AC signal must be transmitted between the telephone line and the circuit of the modem unit.
Fourth, although the AC signal on the side of the telephone line is comprised of balanced signals, the signals are processed as unbalanced signals on the side of the computer so that the circuits can be made in the form of an IC. Consequently, it is necessary to match the balanced and unbalanced signals.
Fifth, the signal which is transmitted on the telephone line is mixed with in-phase noise within the transmission path, and this in-phase noise must be eliminated.
The five requirements described above can easily be satisfied by the use of a line transformer. For this reason, the conventional modem unit is provided with a line transformer.
FIG. 1 shows an example of the conventional modem unit. In FIG. 1, a ring detection circuit 102 is connected to a telephone line 101 which connects to an exchange 100. The ring detection circuit 102 detects a calling signal from a remote end. Hook switches 103 respond to the on-hook/off-hook of the receiver. A line transformer 104 couples a primary side line (telephone line side) and the secondary line side (modem circuit side) with respect to the AC signal but provides a DC isolation between the two.
A modem circuit 105 converts an analog signal into a digital signal, and converts a digital signal into an analog signal. An interface 106 on the side of the modem unit couples the modem circuit 105 and an interface 107 of a data terminal equipment 108. The interface 107 on the side of the data terminal equipment 108 couples the data terminal equipment 108 and the interface 106 of the modem unit.
When the calling signal is input from the exchange 100 via the telephone line 101, the ring detection circuit 102 operates and inputs the calling signal to the modem circuit 105. The modem circuit 105 closes the hook switches 103 in response to the calling signal. The AC signal on the primary side line which is received from the telephone line 101 is transmitted to the secondary side via the line transformer 104. Hence, the analog signal is converted into the digital signal in the modem circuit 105, and the digital signal is input to the data terminal equipment 108 via the interfaces 106 and 107.
When transmitting data from the data terminal equipment 108 to the telephone line 101, the modem circuit 105 closes the hook switches 103 and makes a data transmission request with respect to the remote end. When the remote end answers, the modem circuit 105 converts the digital signal into the analog signal which is an AC signal, and this AC signal is transmitted to the primary side line via the line transformer 104. In addition, this AC signal is transmitted to the remote end via the telephone line 101.
A description will now be given of the transformer function of the conventional modem unit shown in FIG. 1, by referring to FIGS. 2(A) through 2(C).
FIG. 2(A) shows a case where a line transformer is used in the modem unit, and the illustration of other circuit parts is omitted. When a line transformer 120 is used, all of the five requirements described above are satisfied.
In other words, the DC loop is formed by the primary side line and the first demand is satisfied. The DC isolation between the primary side line and the secondary side line is achieved because the line transformer 120 is used, and the second demand is satisfied. The transmission of the AC signal is achieved because the line transformer 120 is used, and the third demand is satisfied. Matching of the balanced and unbalanced signals can also be accomplished since the line transformer 120 is used, and the fourth demand is satisfied. Furthermore, noise elimination can be made and the fifth demand can be satisfied for the following reasons.
Specifically, the noise which is mixed on the telephone line in the transmission path is transmitted via two wires, as in-phase signals, where the telephone line is comprised of two wires L1 and L2. For this reason, the in-phase signals when input to the line transformer 120 cause magnetic fluxes to cancel each other, and the noise component will not be transmitted to the secondary side of the line transformer 120.
Therefore, the line transformer 120 can easily satisfy the demands with respect to the modem unit. However, there is a problem in that the line transformer 120 is bulky because of the need for a DC current to flow, which is necessary to capture the line.
Accordingly, it is possible to provide a DC current loop independent of the line transformer as shown in FIG. 2(B), so that no DC current flows to the line transformer.
In FIG. 2(B), a repeater transformer 130 does not supply a DC component to the primary side and transmits only the AC component to the secondary side. A capacitor 131 is provided to cut off the DC component, and a loop coil 132 is provided to supply a DC current from the telephone line (L1, L2).
The repeater transformer 130 also satisfies the first, third, fourth and fifth demands of the modem unit, similarly to the line transformer 120. In addition, the second demand of the modem unit is satisfied by the loop coil 132.
However, there is a problem in that the size of the loop coil 132 cannot be reduced satisfactorily.
FIG. 2(C) shows a case where the DC current loop is formed by an active element. In a pseudo inductance circuit 140 shown in FIG. 2(C), a diode bridge 141 is provided so that the polarity of the operating voltage of a pseudo inductance 140 is maintained constant even if the polarity of the telephone line (L1, L2) is inverted. A capacitor C1 is provided to bypass the AC component so that the bias voltage of transistors TR1 and TR2 does not deviate by the AC component. Voltage dividing resistors R1 and R2 determine the bias voltage of the transistors TR1 and TR2. The transistors TR1 and TR2 form the DC current loop. An emitter resistance R3 forms the DC current loop together with the transistors TR1 and TR2.
A coupling capacitor C2 cuts off the DC voltage and passes only the AC component. In addition, a repeater transformer 142 is provided.
By the DC voltage biasing, an approximately constant DC current flows through the transistors TR1 and TR2 and the resistance R3 without being affected by the AC component. The DC voltage is cut off by the capacitor C2 and only the AC component is supplied to the primary side of the repeater transformer 142 and transmitted to the secondary side.
However, the conventional modem unit requires the line transformer, the repeater transformer, the loop coil or the like which are all bulky. Thus, the size of the modem unit cannot be reduced satisfactorily. As a result, it is impossible to accommodate all circuit parts of the modem unit including the transformer, within an IC card.
FIG. 3 shows another example of the conventional modem unit. A modem unit 200 basically corresponds to the modem unit shown in FIG. 2(C).
In the modem unit 200 shown in FIG. 3, input protection elements 201 protect the modem unit 200 from a high impulse voltage caused by a lightening or the like. A diode bridge 202 always maintains the polarity of the DC voltage which is applied to a modem circuit 210 constant even if the polarity of the DC voltage on the telephone line is inverted. An off-hook/dialing circuit 203 detects the off-hook and outputs a dial signal. A ring detection circuit 204 is comprised of a photocoupler 205, and detects the calling signal. The photocoupler 205 optically detects the calling signal.
A pseudo inductance circuit 206 bypasses the DC current from the telephone line. The pseudo inductance circuit 206 includes a capacitor Cl, voltage dividing resistors R1 and R2, transistors TR1 and TR2, and coupling capacitors C2 and C3 for cutting off the DC current and passing the AC signal.
A line transformer 207 provides a DC isolation between the telephone line and a terminal equipment 211, and transmits the AC signal. A DC/DC converter 208 shifts the DC voltage on the telephone line, and supplies the shifted voltage to a modem circuit 210 as a power source voltage. A transformer 209 provides a DC isolation between the terminal equipment 211 and the telephone line. The modem circuit 210 modulates the output digital signal of the terminal equipment 211 into the analog signal, and also demodulates the analog signal from the telephone line into the digital signal.
When a call is made from the remote end, the ring detection circuit 204 is activated and the calling signal is detected by the photocoupler 205. When the calling signal is detected, the modem circuit 210 is notified.
The pseudo inductance circuit 206 bypasses the DC current from the telephone line. More specifically, the DC voltage is divided by the resistors R1 and R2 into the bias voltage of the transistors TR1 and TR2 which form a Darlington pair. The capacitor C1 presents the bias voltage of the transistor TR1 from deviating due to the AC component. Hence, an approximately constant current flows from the collector to the emitter of each of the transistors TR1 and TR2 regardless of the magnitude of the AC component. The DC current is cut off by the capacitors C2 and C3. For this reason, the DC current will not flow to the line transformer 207, and only the AC signal is input to the line transformer 207 and transmitted to the modem circuit 210.
The modem circuit 210 converts the output digital signal of the terminal equipment 211 into the analog signal, and transmits this analog signal to the telephone line via the line transformer 207, the capacitors C2 and C3 and the pseudo inductance circuit 206.
The DC/DC converter 208 subjects the DC voltage from the telephone line to a DC/DC conversion, and supplies the DC voltage to the modem circuit 210. The transformer 209 of the DC/DC converter 208 guarantees the DC isolation between the telephone line and the terminal equipment 211.
The off-hook/dialing circuit 203 is comprised of a photocoupler or the like, and transmits a dial signal by detecting an off-hook signal.
This modem unit 200 also uses the line transformer 210 which is bulky. For example, the line transformer 210 has a height of approximately 10 mm. Therefore, the modem unit 200 including the line transformer 210 cannot be made in the form of an IC circuit.